1. Field of the Invention
The present invention relates to an inverter transformer for use in an inverter circuit to light a discharge lamp, such as a cold cathode fluorescent lamp, as a light source of a lighting device for a liquid crystal display.
2. Description of the Related Art
Currently, a liquid crystal display (LCD) is increasingly used as a display unit for a personal computer, and the like. The LCD lacks a light emitting function, and therefore requires a lighting device, such as a back-light system or a front-light system, and a cold cathode fluorescent lamp (CCFL) is generally used as a light source for such a lighting device. In case of discharging and lighting a CCFL having a length, for example, about 500 mm, an inverter circuit is used which is adapted to generate a high-frequency voltage of 60 kHz, about 1600 V at the time of starting discharge. The inverter circuit controls a voltage applied to the CCFL such that after the CCFL is discharged, the voltage is lowered to about 1200 V which is a voltage required for keeping the discharge. Some inverter circuits include a closed magnetic path type inverter transformer and also a ballast capacitor, and the ballast capacitor additionally required prohibits reduction in dimension and cost. Further, even after discharging a CCFL, the voltage at the time of starting discharge must be maintained, which is disadvantageous in view of safety.
Recently, an open magnetic path type inverter transformer is employed which leverages the function of a leakage inductance serving as a ballast capacitance in place of a ballast capacitor. Some of such open magnetic path type inverter transformers may use a bar-shaped magnetic core (I-core), and others may use a combination of a bar-shaped magnetic core and a rectangular frame-shaped magnetic core (refer to Japanese Patent Application Laid-Open No. 2002-353044).
FIG. 19 is an equivalent circuit of an inverter transformer having a leakage inductance as described above. Referring to FIG. 19, the inverter transformer includes an ideal transformer 1 having no loss with a winding ratio of 1:n, leakage inductances L1 and L2, and a mutual inductance Ls, and CCFLs 2. In the inverter transformer, the leakage inductances L1 and L2 function as a ballast inductance, and the CCFLs 2 can be lighted normally without using a ballast capacitor.
FIG. 20 is a schematic view of a traditional inverter transformer 1A of open magnetic path type. The inverter transformer 1A includes a bar-shaped magnetic core (I-core) 3 indicated by a dashed line, a bobbin 4 defining a hollow 5 to house the bar-shaped magnetic core 3, a primary winding 6 wound around the bobbin 4, a secondary winding 7 wound around the bobbin 4, a terminal block 9 provided with terminal pins 8 for the primary winding 6, and a terminal block 11 provided with terminal pins 10 for the secondary winding 7. Since a high voltage is induced at the secondary side, the secondary winding 7 is divided by partitions 12 formed at the bobbin 4 in order to prevent surface discharge. The inverter transformer 1A in FIG. 20, which employs a bar-shaped magnetic core as described above, is simple in structure compared with an inverter transformer (not shown) which employs a magnetic core having a closed configuration, such as a rectangular core. However, magnetic flux leaks from the bar-shaped magnetic core, especially from the ends thereof
FIG. 21 is an exploded perspective view of another traditional inverter transformer 1B. The inverter transformer 1B includes a bar-shaped magnetic core 3, a rectangular frame-shaped magnetic core 13, a bobbin 14 having a hollow to house the bar-shaped core 3, and primary and secondary windings 6 and 7 wound around the bobbin 14. The end portions of the bar-shaped magnetic core 3 are engaged with respective recesses 15 of the rectangular frame-shaped magnetic core 13 such that gap sheets formed of a non-magnetic material are put between the bar-shaped magnetic core 3 and the rectangular frame-shaped magnetic core 13 so as to form gaps therebetween, thereby generating a prescribed amount of leakage inductance. In the inverter transformer 1B thus structured, magnetic flux leaking from the bar-shaped core 3 passes through the rectangular frame-shaped magnetic core 13, and leakage flux is small compared with an inverter transformer employing only a bar-shaped magnetic core (without a rectangular frame-shaped magnetic core).
In an inverter transformer involving leakage inductance, leakage flux may possibly influence neighboring components or wires, or emit noises, and the components and wires must be appropriately located in order to keep away from the leakage flux thus placing restrictions on arrangement of components and wires. This may result in increase of product dimension or deterioration of characteristics. Also, if a magnetic material is placed at the path of the leakage flux, the flux path may be influenced when the leakage flux passes through the magnetic material, which causes the leakage inductance to vary or fluctuate disturbing stability, further causing the inverter transformer to undergo variation in characteristic and consequently to undergo change in operation.
Thus, an inverter transformer including only a bar-shaped magnetic core is simple in structure but suffers increase in leakage flux distribution range, and also has difficulty in adjusting the amount of leakage inductance. On the other hand, an inverter transformer including a rectangular frame-shaped magnetic core together with a bar-shaped magnetic core has a smaller leakage flux distribution range than the inverter transformer including a bar-shaped magnetic core only, but incurs increase in number of components, and a molding or machining process is required for producing the rectangular frame-shaped magnetic core. Also, when engaging the bar-shaped magnetic core with the rectangular frame-shaped magnetic core, a complex and troublesome process of putting gap sheets therebetween is required for adjusting leakage inductance.
An inverter transformer incorporating only a bar-shaped magnetic core generates a wide distribution range of leakage flux as described above. Such an inverter transformer is magnetically shielded in order to prevent the inverter transformer from affecting neighboring components, and also to prevent the neighboring components from affecting the inverter transformer. This solution by magnetically shielding a product, however, requires a shielding case, and this leads to increase in product dimension and product cost. Also, processes of fixing the inverter transformer to the shielding case and taking out lead wires from the shielding case are additionally required, thus making cost reduction further difficult. And, a defective fixing of the inverter transformer to the shielding case may raise deterioration in reliability. On the other hand, an inverter transformer employing a rectangular frame-shaped magnetic core together with a rectangular frame-shaped magnetic core, while generating a reduced amount of leakage flux, has a complicated structure and requires additional troublesome manufacturing processes thus pushing up production cost.